1. Field of the Invention
The present invention relates to a noise reduction technique for reference pixels for black level (OB pixels) in an image capturing apparatus.
2. Description of the Related Art
Recently, advances in image sensors have brought remarkable developments in image capturing apparatus such as electronic still cameras and video cameras. An image sensor typified by a CMOS (Complementary Metal-Oxide Semiconductor) image sensor includes a pixel array in which a plurality of pixels are arranged in the row and column directions, and is composed of an effective pixel region in which photosensitive pixels are arranged and an OB region in which light-shielded reference pixels for black level (OB pixels) are arranged. An image capturing apparatus using such an image sensor includes an OB clamping circuit, which uses OB pixels to remove DC components of dark currents, which greatly vary with variations in conditions such as temperature, and low-frequency variations in signals accompanying power supply variations.
It is theoretically possible to use a vertical (V) OB or horizontal (H) OB as a clamp reference (reference for black level) for such OB clamping. This is, however, based on the assumption that normal pixel outputs are output from an OB region regardless of the system of the OB clamping circuit. If a so-called pixel defect is present in the OB region, clamp information contains information other than proper OB pixel information, resulting in a clamping error. This causes image quality deterioration. When HOB clamping is employed, in particular, a clamp error (clamp offset) occurs on only the target line, which differs in level from the upper and lower lines. This signal level difference appears as horizontal streak noise, which is very noticeable. That is, the presence of a slight defect becomes a factor of poor image quality.
In addition, noise caused by an OB pixel is a factor of horizontal streak noise. It is therefore necessary to perform HOB clamping by using as many OB pixels as possible. For this purpose, the following methods have been proposed. The technique disclosed in Japanese Patent Laid-Open No. 2001-268448 is a method of preventing image quality deterioration caused by OB pixel defects at the time of OB clamping within the range of the operating conditions of an image capturing apparatus (temperature and exposure time) by setting a stricter defect determination level, regarding pixel defects in an image sensor, in an OB region than in a normal effective pixel region. However, this method sets a stricter defect determination level in an OB region than in a normal effective pixel region, and hence decreases the yield of image sensors, resulting in an increase in cost.
In addition, according to the technique disclosed in Japanese Patent Laid-Open No. 2002-064196, in an image sensor including an OB region composed of first OB pixels having photo-electric converters and second OB pixels having no photo-electric converters, this method performs stable HOB clamping by using the second OB pixels including no pixel defects caused by the photo-electric converters, and removes DC components of dark currents by using the average value of the first OB pixels in a signal processing circuit. Furthermore, Japanese Patent Laid-Open No. 2003-134400 discloses an arrangement obtained by applying an arrangement similar to the OB region in Japanese Patent Laid-Open No. 2002-064196 to a CMOS image sensor.
An image capturing apparatus using a CMOS image sensor includes a noise removal circuit which includes a sample/hold circuit and switch transistor for each vertical signal line to remove pixel irregularity caused by threshold variations among the amplification transistors of pixel amplifiers for the respective pixels. However, threshold variations among the transistors in circuits provided for the respective vertical signal lines cause different kinds of noise on the respective columns, resulting in vertical streak noise on a reproduced image, thus causing a new problem.
In order to remove this vertical streak noise, the following method has been proposed. The technique disclosed in Japanese Patent Laid-Open Nos. 2000-261730 and 2006-025146 is a method of removing vertical streak noise by generating a one-line correction signal by adding a plurality of OB line signals read from a VOB region for each column and subtracting the correction signal from an effective pixel line signal from an effective pixel region.
With the recent trend toward the larger number of pixels and advances in semiconductor microfabrication techniques, image sensors have tended to have smaller pixel areas. Along with this tendency, each element included in each pixel tends to be smaller. Consider, for example, a case of the miniaturization of an amplification transistor of each pixel amplifier, which is a MOS transistor for amplifying a signal corresponding to the electric charge generated by a corresponding photo-electric converter.
Letting W be the gate width of an amplification transistor, L be the gate length, and Cox be the gate insulating film capacitance per unit area, it is known that the noise generated by the amplification transistor is inversely proportional to the square root of (W×L×Cox). That is, as the amplification transistor is reduced in size to decrease the gate width or gate length, the random noise generated by the amplification transistor increases. Consequently, even in an image capturing apparatus using a CMOS image sensor, as described above, reducing the pixel area will increase the random noise generated by the amplification transistors of the pixel amplifiers in the effective pixel region and OB region.
According to the Japanese Patent Laid-Open Nos. 2002-064196 and 2003-134400, since clamping operation is executed by using reference signals for black level read from the HOB region, as the amount of noise generated by the amplification transistors of the pixel amplifiers and included in the reference signals for black level increases, the clamping precision decreases. This causes image quality deterioration. Although this problem is reduced by increasing the number of pixels in the HOB region, the area of the image sensor undesirably increases, resulting in another problem.
The technique disclosed in the Japanese Patent Laid-Open Nos. 2000-261730 and 2006-025146 corrects vertical streak noise in signals read from the effective pixel region by using the reference signals for black level read from the VOB region. For this reason, as the amount of noise generated by the amplification transistors of the pixel amplifiers and included in the reference signals for black level increases, the correction precision deteriorates, leading to image quality deterioration. Although the precision of correction signals can be improved by increasing the number of lines in the VOB region to be used, a general image sensor has an angle of view in landscape orientation. For this reason, increasing the number of lines in the VOB region will have greater influence on an increase in the area of the image sensor than increasing the number of pixels in the HOB region.
In clamping using HOB pixels, it is possible to use signals from the HOB pixels, which are read before lines including the HOB pixels for clamping, for integration for clamping. However, use of only VOB pixels equal in number to the number of lines in a VOB region is allowed for the correction of vertical streak noise because of addition processing for each column. It is therefore difficult to obtain a sufficient number of VOB pixels for the generation of correction signals for vertical streak noise.